The present invention relates to a suspension for vehicles, in particular road vehicles. It relates both to the guidance of a vehicle wheel relative to the body of this vehicle and to the organization of the deflection of the wheel relative to the chassis. It relates more particularly to means used for actively controlling the position of the wheel plane relative to the body.
It is known that a wheel is mounted on a hub and that the hub is mounted on a wheel carrier by means of a rolling bearing which embodies the axis of rotation of the wheel. The guidance of the wheel involves allowing the wheel carrier to be displaced relative to the vehicle by an extent which is sufficient for the suspension of the vehicle. Moreover, the guidance of the wheel must be such that the control of the path of the vehicle via the grip of the tires on the ground is ensured under the best possible conditions. The deflecting movement which is sought is of great extent in the vertical direction.
It is customary for this movement to be designated by the expression "vertical suspension". It may be desirable to allow slight movement in the longitudinal direction ("horizontal suspension" is then referred to), while, ideally, the aim is to avoid any relative movement in the transverse direction. Moreover, the steering and the camber variations which all these movements may induce must be strictly contained, thus also resulting in a design constraint which the average person skilled in the art must take into account.
If the "plane" of the wheel is what is meant by the plane perpendicular to the axis of rotation of the wheel and passing through the middle of the wheel or, more specifically, through the center of the area of contact with the ground of a tire mounted on said wheel, the control of the deflecting movements is tantamount to saying that guidance must ensure a strictly controlled position of the plane of the wheel relative to the chassis. It is still possible to define a longitudinal and vertical plane forming a reference to identify the preferred running direction which is parallel to said longitudinal plane. The purpose of guiding the plane of the wheel is to control as strictly as possible the relative position of the plane of the wheel relative to said longitudinal plane in terms of angle and distance.
The guidance of the wheel plane directly affects the attitude of the tire in relation to the ground, hence the stresses which said tire undergoes and the more or less favorable position which it assumes in order to transmit forces, in particular forces in the transverse direction. Now these forces are known to be of primary importance for the guidance of the vehicle and therefore for safety.
Road vehicles in use at the present time are known to experience a rolling movement precisely because of their suspension. In fact, the centrifugal force causes a transfer of load toward the outside of the bend, thus resulting in an increase in compression of the suspensions on the outside of the bend and an expansion of the suspensions on the inside of the bend. Now this roll is detrimental both to passenger comfort and to the roadholding of the vehicle due to the increase in the camber of the wheels which it brings about. The means used most often to combat this roll involves using one or more antiroll bars. Unfortunately, an antiroll bar can only limit the roll by opposing a reaction torque to the latter. In principle, it cannot prevent roll from taking hold, and therefore it cannot prevent the wheels from being inclined in a direction unfavorable to the proper functioning of the tires.
Other solutions making it possible to combat roll more radically are also known. It is possible to conceive a pendulum suspension (purely passive). Such suspensions can force the wheel plane to be inclined toward the inside of the bend by means of a suitable mechanism, or else they can also transfer the rolling axis of the vehicle above the center of gravity, so that the vehicle body is naturally inclined toward the inside of the bend. However, pendulum suspensions have not had much success, probably because of the difficulty of installing suspension arms and joints making it possible to achieve this result. The intrusion of suspension arms and joints in places which it is desirable to leave free for the passenger compartment or for installing the engine or transmission of the vehicle would appear to be a disadvantage. This problem seems to limit this type of suspension to a few types of vehicle, such as, for example, in the field of railroads. Moreover, in general terms pendulum suspensions increase the overload on the wheels located on the outside of the bend by displacing the center of gravity of the vehicle toward the outside of the bend. This is particularly detrimental with regard to road vehicles equipped with tires.
Moreover, research for an even better compromise between suspension comfort and roadholding leads to the provision of so-called active or semiactive suspensions, the control of which is now made possible by advances in electronics. However, it is found that these control means are transplanted onto suspensions which are still of traditional mechanical design, said control means being adapted to the defects of these suspensions, without looking to make use of new parameters in suspension kinematics. In other words, active or semiactive suspensions are concerned simply with controlling the damping characteristics of the suspensions and even the flexibility characteristics, while at the same time preserving suspension kinematics derived directly from conventional forms of construction.